1. Field of the Invention
This invention relates to circular knitting machines and in particular to the selection of needles in such machines for the purpose of producing patterned knitwork, and provides a device and method for selecting those needles which are to pick up the yarn from the feeds to form knitwear articles.
2. Description of the Related Art
Circular knitting machines are known to consist essentially of one or two needle cylinders which, as shown in FIG. 1, are provided with tricks 1 in their outer cylindrical surface. The tricks represent the guides for the needles 2 which during their travel form the stitch loops in cooperation with the sinkers.
The number of tricks is equal to the number of needles 2 which slide reciprocatingly in them.
Generally, in hosiery machines the number of tricks and needles is between 200 and 400 per cylinder.
The needles operate with reciprocating movement between a maximum position and a minimum position into which they are moved by suitable cams acting on the needle and jack butts.
The cylinder is rotated and with it there rotate the needles which during their reciprocating movement are fed with yarn in an angularly fixed position when in their highest point of travel. To produce hosiery articles generally only part of the available needles are used at the same time and in the same manner, except for the plain knitwork parts, for which all the needles are operated between their maximum and minimum level, all being fed with yarn at each knitting course, and all being moved in the same manner.
When the machine is not producing plain knitwork, in order to produce other types of knitwork (such as mesh or patterned knitwork) some needles are required to produce stitch loops while others have to be raised to an intermediate level to take up yarn without clearing the previous stitch in order to form a tuck stitch, or have to be raised with a certain delay so that they do not pick up the yarn fed into a certain angular position and therefore do not form new loops with it. In other words a needle selection has to be made. This means that for each feed it has to be determined which and how many needles must undergo a certain travel and which and how many other needles must undergo a certain different travel or indeed undergo no travel.
Again with reference to the arrangement shown by way of example in FIG. 1, this selection is made by the jacks 3 which slide in the same tricks 1 as the needles lying above them, to push these latter upwards to a higher level in order to seize the yarn. FIG. 1 shows an elastic jack 3, able to radially inflect its lower end.
The needles 2 are driven reciprocatingly by cams 4 and fixed counter-cams 5 which cause them to move to form the stitch loops. When the jacks 3 have moved the needle into its working position they withdraw from the needle butt and return downwards. If the needle, after completing its task of seizing the yarn and forming the stitch loop and therefore being at its minimum level, is not required to pick up a further yarn from another feed it remains at this level because its control jack remains in its lower rest position.
The jack 3 has a special shape which corresponds to a precise function.
Although not shown on the drawings, it is slightly curved, or bowed, in a direction perpendicular to the plane of FIG. 1. This curvature keeps the jack lightly forced towards the inside of the trick and ensures its accurate positioning and lack of vibration by keeping it properly adhering to the trick walls, but requiring the application of a certain force to move it either axially or radially.
The shank 13 of the jack comprises in its middle part a projection 6, i.e. the upper guide butt, which comes into engagement with its own control cam 7 for urging the jack downwards when it has completed its task of pushing the needle 2.
Proceeding downwards along the jack shank there is a lower butt 8 which comes into engagement with the cam ring 9 provided with a raising contour 10 which raises the jack together with its overlying needle, this therefore being selected to seize the yarn, and with an inner face 11 which causes the elastic jack to approach the interior of the trick.
The lowering cam 7 and the raising and approach contours 10, 11 are obviously offset angularly and operate at different times on each jack.
The elastic jacks 3 tend spontaneously to move their lower butt 8 outwards to engage the raising contour 10 and be raised.
The jacks 3 are maintained in position by one or more circular springs 12 which surround their upper part. The springs themselves are held in position by one or more circumferential grooves in the cylinder, so that the springs lie internal to the face of the needle cylinder. The springs 12 must be sufficiently strong to oppose the forces which flex the shank 13 of the jack so that the flexural forces acting on the lower butt 8 induce said flexure, so that the upper part of the jacks does not leave the tricks. The purpose of the selection mechanism and procedure is to exclude from this totality of jacks the jacks which control those needles which in forming the particular stitch are not required to be raised.
In the known art, as disclosed for example in GB Patent 214,704 and U.S. Pat. No. 4,716,743 the mechanism for selecting or inactivating the needles consists of a plurality of levers or slides which come into contact with a plurality of pattern butts on the lower part of the jack, in an intermediate position between the upper and tower butts 6 and 8, and which urge the jack back into the trick 1 so preventing it making contact with the raising cam 10.
The traditional selection procedure as disclosed in said prior art documents consists of bringing a certain number of slides or levers into contact with a certain number of jacks 3 via the pattern butts located at the same height, by radially moving only some of the slides towards the outer surface of the cylinder. If a determined jack is to be left engaged when one or more of the slides have approached the needle cylinder, the butts corresponding to the height of those levers are removed from the jack. The number of levers or slides available for selection control is generally equal to the number of available pattern butts.
The selection procedures therefore generally consist of producing contact between the non-removed pattern butts of the jacks and the inactivating members, whether levers or slides, by rotating said inactivating members into a position of approach to the cylinder.
Obviously those inactivating members, which are not required to inactivate the jacks whose pattern butts are in a position corresponding with them, are kept in the retracted position at the moment in which they would have made contact.
Needle selection by mechanical devices places very restrictive limits on the machine speed and the possible sequence combinations of needles in their raised position and needles in their lowered position.
Recently proposed solutions are based on electromagnetic selection, using fixed electromagnetic selection devices.
These fixed needle selection devices operate on the jacks which raise the needles into activation when said jacks, during their rotation together with the cylinder, appear in front of the fixed selection station which precedes each machine feed station. The time available for setting, initiating and completing the selection is very small, being of the order of a few thousandths of a second, and determined by the small angular sector within which the rotating jacks face the selection member, which for its part must be immediately ready to select those needles or more precisely those needle jacks which at that moment are presented to them.
Most recently, the solution to the problem has turned towards mobile selection devices rotating together with the cylinder, so that the time available for selection is not limited to the moment in which the jacks appear before the stationary selection device. In this manner each jack is constantly presented to its selection member, so that the selection can take place within a wide angle of the cylinder rotation. In this manner the selection setting time is not so drastically small and the selection can be effected reliably and safely.
In EP Publ. No. 0 379 234 a needle selection is disclosed, which is effected by controlling the radial position of the jacks by means of other horizontal jacks which slide radially. These horizontal jacks are selected by electromagnetic devices, by being caused to assume a position withdrawn from the cylinder to thus allow the corresponding needle to operate, or a position close to the cylinder to thus inactivate the needle.
EP Public. No. 0 441 005 proposes to control the radial position of said horizontal jacks by axially deformable elastic forks which rotate together with the cylinder.